Recently, aircraft propulsion turbines have been developed having a pair of counter rotating external unshrouded fans. Pitch adjustment in turbines with unshrouded fans is necessary for the safe and efficient operation of the aircraft throughout the flight envelope.
Aircraft turbine propulsion engines with external unshrouded fans in the 20-30,000 horsepower class require 20 or more net horsepower per blade section to achieve the required pitch adjustment change rates at the operating loads. Current designs of unshrouded fan turbines change pitch of the blades by a hydraulic pump-power drive unit combination. The hydraulic pump is driven by power taken by direct mechanical connection to the turbine main shaft to produce pressurized hydraulic fluid which is applied to the remotely located power drive unit by means of hydraulic lines. The pitch adjusting mechanism has an outer shaft which is driven by a free turbine powered by pressurized gas after passage through the turbine section powering the main shaft. The free turbine directly supplies the rotary power for the unshrouded fans and therefore the rotational velocity of the free turbine is directly proportional to the rotational velocity of the blades. The pitch adjusting mechanism also has an inner control shaft concentric with the outer shaft which rotates at a rotational velocity proportional to a charge in pitch specified by a pitch control signal source which produces a signal under pilot or automatic control. A transmission provides the reference and control shafts to the pitch adjusting mechanism. The transmission receives power directly from the rotating blades of the unshrouded fans and also receives an input from the power drive unit which has a rotary velocity proportional to the change in pitch control signal.
The magnitude of the hydraulic horsepower required to vary the pitch of the unshrouded fans is several times that required to power conventional hydraulic loads on aircraft. Because of the intermittent requirements for changing pitch of external unshrouded fans in jet aircraft, the prior art approach of taking hydraulic power from the main hydraulic power supply system to drive the pitch adjusting mechanisms results in extra hydraulic lines and could result in an overall weight penalty. It is necessary with the prior art to size the main hydraulic power system to handle the intermittent hydraulic power demands of the pitch adjusting mechanism and the other conventional loads.
U.S. Pat. Nos. 4,660,437; 4,261,226; 3,994,128; 3,893,789 and 3,395,763 disclose mechanisms for adjusting the pitch of a propeller.